This invention relates to an ink additive for lithographic printing.
Lithographic printing plates consist of greasy ink receptive oleophilic image portions and ink repellent oleophobic non-image portions, the latter being generally water receptive hydrophilic areas. Therefore, the customary lithographic printing is carried out by feeding water and colored ink to the printing plate surface to allow the image portions to receive preferentially the colored ink and the non-image portions preferentially water and then transferring the ink deposited on image portions onto a substrate such as paper. As lithographic printing plates which use a damping solution, there have been known presensitized plates (so-called PS plate) which use diazonium compounds, lithographic printing plates made electrophotographically using zinc oxide or organic photoconductors and lithographic printing plates made by silver salt photography which uses a silver halide emulsion as a light sensitive component. There are a wide variety of inks used for these lithographic printing plates. However, conventional printing inks can provide good prints in case of using some specific printing plates or damping solutions, but they sometimes cause various defects on change of kinds of the printing plates or printing conditions and often cannot be used at all. As properties required for such printing ink, not only deposition of sufficient ink on image portions and deposition of no ink on non-image portions are important, but also flowability, interfacial property and drying characteristics must also be considered. Especially, in the lithographic printing, oily ink and water are repeatedly fed for every print during printing of thousands to tens of thousands of copies and hence image portions are caused to expand or to disappear, emulsification of ink occurs or scumming occurs unless interfacial tension or surface tension between oily ink and water is properly balanced. Therefore, oily inks of excellent interfacial properties are required.
U.S. Pat. No. 4,589,920 discloses an ink additive for lithographic printing which comprises organopolysiloxane having hydrophilic group and Japanese Patent Kokai No. 61-138677 describes an additive for lithographic printing ink which comprises a compound having alkylene oxide unit and having an HLB of 3-13. This compound can give good interfacial properties to ink and prevent scumming, but they are not sufficient for some lithographic printing materials, inks, damping solution and printing machines. For example, when a printing plate made by silver salt photography which is poor in water retention is used for high speed offset rotary printing, scumming occurs much and when the compound described in the above Japanese patent publication is added to ink, the scumming can be reduced, but this is not sufficient. Further, it is well known to use fine particles of 0.1.mu. or less such as colloidal silica in solutions for treatment of plate surface subsequent to development treatment in making of the lithographic printing plate by silver salt photography, for example, damping solution disclosed in Japanese Patent Kokai No. 48-45305, neutralizing solution in Japanese Patent Kokai No. 54-83502, neutralizing solution, ethch solution and damping solution in Japanese Patent Application No. 55-104176 and fixing agent (fixing solution) and damping solution in Japanese Kokoku No. 45-29001. It is considered that inorganic fine particles such as colloidal silica and colloidal alumina are adsorbed onto the surface of lithographic printing plate to form a hydrophilic layer on the surface, but the object of prevention of stain by printing ink which is most desired cannot be sufficiently accomplished and besides these particles damage the ink receptivity of image portions.
Thus, development of techniques according to which staining can be prevented without using such inorganic fine particles has been much demanded.